PROCESS FOR DRYING GRANULAR POLYMERIC MATERIAL AND PLANT OPERATING ACCORDING TO SAID PROCESS

Abstract

A process for drying granular polymeric material, includes: dehumidifying the granular polymeric material by a first flow of gas at a first temperature of between 100° C. and 150° C.; heating the dehumidified granular polymeric material to a second temperature, greater than the first temperature; drying the granular polymeric material heated to the second temperature, by applying a predefined vacuum level.
The pressure-sealing elements include a filling unit, which includes a small tank blocked upstream and downstream by shut-off valves, as well as a discharge unit, which includes a small tank blocked upstream and downstream by respective shut-off valves.

Claims

1. A process for drying granular polymeric material, comprising: dehumidifying said granular polymeric material by a first flow of gas introduced into said granular polymeric material; heating said dehumidified granular polymeric material to a second temperature that is greater than said first temperature; drying said granular polymeric material heated to said second temperature by applying a predefined vacuum level; wherein said drying process is carried out in a drying hopper that is separated from the other hoppers, both upstream and downstream, by pressure-sealing elements wherein said pressure-sealing elements comprise a filling unit, which includes a small tank blocked upstream and downstream by shut-off valves, as well as a discharge unit, which includes a small tank blocked upstream and downstream by respective shut-off valves.

2. The process according to claim 1, wherein said granular polymeric material is dehumidified at a first temperature of between 100° C. and 150° C.

3. The process according to claim 1, wherein said first flow of gas is air that is taken from the environment and is not recirculated and is heated using a heat pump.

4. The process according to claim 1, wherein said granular polymeric material is heated to said second temperature by a second flow of gas introduced into said granular polymeric material by a circuit for recirculating said second flow of gas.

5. The process according to claim 4, wherein said drying step is carried out in a drying hopper, and wherein said granular polymeric material is transported by said second flow of gas.

6. The process according to claim 1, wherein said granular polymeric material is subjected to a post-heating step either during or after said drying step.

7. The process according to claim 1, wherein said dried granular polymeric material is transferred to a feed hopper that is provided upstream of a machine for working said granular polymeric material.

8. The process according to claim 6, wherein said dried granular polymeric material is transferred to a feed hopper that is provided upstream of a machine for working said granular polymeric material.

9. The process according to claim 8, wherein said post-heating step is carried out in said feed hopper.

10. The process according to claim 9, wherein said granular polymeric material is post-drying heated in an inert atmosphere to a temperature above a maximum temperature at which it can be maintained in air.

11. The process according to claim 10, wherein said granular polymeric material is post-drying heated in an inert atmosphere to a temperature with a value which is less than 50° C. lower than the melting temperature of said granular polymeric material.

12. A plant for drying granular polymeric material, comprising: at least one dehumidification hopper, which is connected to a dehumidification line, through which a first flow of gas for dehumidifying said granular polymeric material is introduced into said dehumidification hopper; at least one heating hopper, which is arranged downstream of said dehumidification hopper and is provided with a heating unit, for heating said granular polymeric material to a second temperature that is higher than the temperature of said first flow of gas; at least one drying hopper, which is provided downstream of said heating hopper and is connected to a depressurization circuit, for obtaining a specified vacuum level in said drying hopper and for drying said granular polymeric material; wherein said drying hopper is separated from the other hoppers, both upstream and downstream, by pressure-sealing elements comprising a filling unit, which includes a small tank blocked upstream and downstream by shut-off valves, as well as a discharge unit, which includes a small tank blocked upstream and downstream by respective shut-off valves.

13. The drying plant according to claim 12, wherein at least one feed hopper is provided downstream of said drying hopper and upstream of a machine for working said granular polymeric material.

14. The drying plant according to claim 12, wherein said drying hopper has a volume of between 500 and 1000 liters and said tank of the filling unit has a volume of between 30 and 50 liters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The characteristics and advantages of the invention will be more clearly apparent from the detailed description of a preferred exemplary embodiment thereof, illustrated by way of example and non-restrictively, with reference to the attached drawing, in which FIG. 1 is a schematic view of a plant for drying granular polymeric material realized so as to operate according to the process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] With reference to FIG. 1, the reference numeral 1 is an overall indication of a plant for drying granular polymeric material, operating according to the process of the present invention.

[0060] The plant 1 is designed to dry any granular polymeric material, for example polyamide, polycarbonate or ABS copolymer, even though, in the specific example described here, the material treated is formed of PET (polyethylene terephthalate) granules.

[0061] PET has a melting point of around 260° C. and a maximum temperature at which it can be maintained in air, as generally provided by the producers, of around 180° C.

[0062] The plant 1 is designed to supply a working machine 100 of the granular polymeric material, which machine, in the specific example, comprises a mold 101 fed by an extruder 102 that injects the polymeric material into the mold 101 in the molten state.

[0063] The plant 1 comprises a dehumidification hopper 10, a heating hopper 20, a drying hopper 30 and a feed hopper 40, all positioned in series with one another. The working machine 100 is placed downstream of the feed hopper 40.

[0064] In the example described here, a single hopper is provided for each step of the drying process; however, two or more hoppers can also be provided in parallel for one or more of said steps.

[0065] By way of example only, for a production capacity of the plant 1 of around 1000 kg/h, the dehumidification and heating hoppers 10, 20 may have a volume of between 1000 and 1500 liters, and the drying and feed hoppers 30, 40 may have a volume of between 500 and 1000 liters.

[0066] The plant 1 comprises a filling unit 2 provided for transferring the granular material from one or more bags 3 of untreated material into the dehumidification hopper 10, via a filling line 4. The bags 3 can contain the same material, or different polymeric materials.

[0067] The filling unit 2 comprises an extractor 5 connected to the filling line 4, and a separation cyclone 6, placed at the top of the dehumidification hopper 10, at which point the granules of polymeric material separate from the transport air flow and are introduced into the hopper.

[0068] The dehumidification hopper 10 is connected to a dehumidification line 11, through which a first flow of gas for dehumidifying the granular polymeric material contained in the dehumidification hopper 10 is introduced.

[0069] The first gas flow is formed by ambient air drawn along the dehumidification line 11 by the action of a fan 12 placed on an outlet pipe 13 of the dehumidification hopper 10.

[0070] A heat pump 14 is provided on the dehumidification line 11, for heating the first flow of gas to a first temperature preferably between 120° C. and 130° C., before feeding it into the dehumidification hopper 10. The first gas flow is distributed into the mass of granular polymeric material to be dehumidified thanks to a diffuser 15 placed inside the dehumidification hopper 10 and, once it leaves the dehumidification hopper 10 by being extracted by the fan 12, is returned to the atmosphere without being recirculated.

[0071] The heating hopper 20 is placed directly under the dehumidification hopper 10, so that the dehumidified granular material can be transferred into the heating hopper 20 by falling directly into it.

[0072] The dehumidification hopper 20 is provided with a heating unit 21, for heating the granular polymeric material to a second temperature, higher than the temperature achieved in the dehumidification hopper 10, for example around 180° C.

[0073] The heating unit 21 comprises a recirculation circuit 22, through which a second flow of gas is fed, also in this case formed of ambient air.

[0074] The recirculation circuit 22 comprises a heating line 23, along which a heater 24 is provided, which heating line enters the heating hopper 20 and emerges into a diffuser 25, conveniently positioned close to the bottom of the heating hopper 20.

[0075] The recirculation circuit 22 further comprises a recovery line 26 leaving the heating hopper 20 and a fan 27 that drives the second gas flow back along the heating line 23.

[0076] A transfer line 28 branches off from the heating line 23 before the heater 24, the transfer line being connected to the bottom of the heating hopper 20 and designed to pneumatically convey the granular polymeric material leaving the heating hopper 20 to an intermediate holding hopper 29, from which a return line 28a starts, carrying the second gas flow back to the fan 27.

[0077] The intermediate holding hopper 29 acts as a small buffer tank from which the drying hopper 30 is fed.

[0078] The drying hopper 30 is connected to a depressurization circuit 31 capable of producing and maintaining a predefined vacuum level inside the drying hopper 30, for example so as to reach a pressure of less than 30 mbar, preferably around 10 mbar.

[0079] The depressurization circuit 31 comprises a vacuum pump 32, connected to a depressurization line 33 in which a pair of filters 34 and a protective condenser 35 are provided.

[0080] Upstream and downstream of the drying hopper 30, a filling unit and a discharge unit for the hopper are provided respectively.

[0081] The filling unit of the drying hopper 30 comprises a small tank 36a, blocked upstream and downstream by respective shut-off valves 36b and 36c, which function overall as pressure-sealing elements.

[0082] Similarly, the discharge unit of the hopper comprises a small tank 37a, blocked upstream and downstream by respective shut-off valves 37b and 37c, which are also designed overall to operate as pressure-sealing elements.

[0083] Such high vacuum levels, equal to an absolute pressure of around 10 mbar, can be achieved thanks to the provision, upstream and downstream of the drying hopper 30, of the tanks 36a and 37a that are in turn sealed by pairs of shut-off valves 36b, 36c and 37b, 37c.

[0084] In the embodiment described here, a microwave irradiation unit 38 is provided in the drying hopper 30, capable of heating the granular polymeric material contained therein.

[0085] Preferably, the microwave irradiation unit 38 comprises one or more Magnetron-type sources that are sufficiently powerful to keep the temperature of the granular polymeric material at the maximum temperature at which it can be maintained in air, for example, in the case of PET, at around 180° C.

[0086] The feed hopper 40 is connected to a circuit for delivering inert gas 41, provided with a fan 42, mounted on an intake line 43 that enters the feed hopper 40, emerging into a distributor 44, and a return line 45 that returns the inert gas leaving the feed hopper 40 to the fan 42.

[0087] A heater 46 is positioned on the feed line 43.

[0088] The feed hopper 40 is connected to the processing machine 100 by means of a discharge pipe 47 fixed to the bottom of the feed hopper 40 by means of a metering valve 48.

[0089] A metering device 49 is also connected to the discharge pipe 47 in order to measure out, if required, any additives to the granular polymeric material that are fed into the processing machine 100.

[0090] The plant 1 operates according to the process described below.

[0091] The granular polymeric material, for example PET, is fed into the dehumidification hopper 10 by means of the filling unit 2, where it is dehumidified by contact with the first flow of air introduced into the dehumidification hopper 10 via the dehumidification line 11.

[0092] The temperature of the first air flow introduced into the dehumidification hopper 10 is around 120-130° C. Once it leaves the dehumidification hopper 10, the first air flow is returned to the environment.

[0093] The dehumidification step lasts around 120 minutes, at the end of which the granular polymeric material has a humidity content of around 1000 ppm and a temperature of around 120-130° C.

[0094] The dehumidified granular polymeric material is then discharged by gravity into the heating hopper 20, where it is brought to the maximum temperature at which it can be maintained in air, equal to around 180° C., thanks to contact with the second air flow fed via the recirculation circuit 22.

[0095] The air introduced into the heating hopper is recirculated without being dried, and thereby the action of dehumidifying the granular polymeric material is overall less comprehensive than the previous dehumidification step.

[0096] At the end of the heating step, the dehumidified and heated granular polymeric material is gradually transferred to the drying hopper 30, using the pneumatic transport provided by the transfer line 28 to the intermediate holding hopper 29.

[0097] From this, the material passes to the filling unit of the drying hopper, the shut-off valve 36b placed upstream of the tank 36a being opened, while the shut-off valve 36c placed downstream of the tank 36a is kept shut.

[0098] The tank 36a is small, for example around 30-50 liters, and the material it contains is transferred to the drying hopper by opening the shut-off valve 36c after having closed the shut-off valve 36b.

[0099] The material is then transferred to the drying hopper 30 a little at a time, to avoid excessive variations in the vacuum level inside the drying hopper 30.

[0100] In the drying hopper 30, the residual pressure is less than 30 mbar, preferably around 10 mbar and this, together with the high temperature, results in effective deabsorption of the humidity present inside the granules.

[0101] After a suitable treatment period, for example around 40-50 minutes, the granular polymeric material has a residual humidity content of less than about 30 ppm.

[0102] During the drying step, the granular polymeric material is post-heated by the microwave irradiation unit 38, to keep the temperature of the material at the temperature of 180° C.

[0103] The dried material is then transferred to the feed hopper 40, passing through the discharge unit and the tank 37a after alternate closing and opening of the shut-off valves 37b and 37c.

[0104] In the feed hopper 40, the dried material can be further post-heated by a flow of inert gas, for example nitrogen, introduced into the feed hopper 40 via the delivery circuit 41.

[0105] The inert gas is introduced at a temperature of around 220-230° C., higher than the maximum temperature at which it can be maintained in air (180° C.) and about 30-40° C. below the melting point of PET (260° C.).

[0106] The granular polymeric material is then transferred to the processing machine 100 through the discharge pipe, actuating the metering valve 48.

[0107] The plant of the present invention can be produced in variations differing from the preferred example described above.

[0108] In a first variant, provision is made not to provide the feed hopper 40 of the delivery circuit with inert gas 41.

[0109] In this case, the granular polymeric material is fed into the processing machine at the maximum temperature at which it can be maintained in air, which the granular polymeric material already has when it reaches the feed hopper 40, thanks to the post-heating carried out by the microwave radiation unit 38.

[0110] In a second variant, provision is made for the delivery circuit 41 to be supplied with air instead of inert gas.

[0111] In this case too, the granular polymeric material is fed into the processing machine at the maximum temperature at which it can be maintained in air.

[0112] In this case, it is possible to heat the granular polymeric material contained in the feed hopper if its temperature tends to fall during its dwell time or if it is not sufficiently heated in the drying hopper, so as to supplement the heating of the microwave irradiation.

[0113] In a third variant, provision is made to eliminate the microwave irradiation unit 38.

[0114] In this case, the post-heating phase takes place only in the feed hopper 40, where it can be carried out with air or inert gas depending on the desired final temperatures.

[0115] Thanks to the process and plant of the present invention, it is possible to obtain excellent results in terms of drying the granular polymeric material while optimizing the energy efficiency of the process.

[0116] Moreover, the plant can change production in a very short period of time, around two hours as against the six hours required in traditional drying plants (with the same production capacity).

[0117] A further important advantage results from the fact that, when the processing machine is fed with a granular polymeric material at a temperature above the maximum temperature at which it can be maintained in air, the energy efficiency of the processing machine is increased.

[0118] Moreover, if the granular polymeric material is fed into an extruder, the latter can be given dimensions with a smaller footprint and power, so as to further improve the layout of the plant as well as the energy efficiency.